An interconnect circuit board is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse type electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. Such a multilayer structure allows a circuit to be more compact.
The use of a ceramic green tape to make low temperature co-fired ceramic (LTCC) multilayer circuits was disclosed in U.S. Pat. No. 4,654,095 to Steinberg. The co-fired, free sintering process offered many advantages over previous technologies. However, shrinkage during the firing process was difficult to control when larger circuits were needed. A trend toward finer via diameters, pitch and lines and spaces continues to push the limit of free sintering LTCC technology.
An improved co-fired LTCC process was developed and is disclosed in U.S. Pat. No. 5,085,720 to Mikeska The process placed a ceramic-based release tape layer on the external surfaces of a green LTCC laminate assembly. The tape controlled shrinkage during the firing process. The process is a great improvement regarding the reproducibility of shrinkage during firing. It allowed the fired dimension of circuit features to be predictable.
U.S. Pat. No. 6,139,666 to Fasano et al. discloses a process where the edges of a multilayer ceramic are chamfered with a specific angle to correct edge distortion, due to imperfect shrinkage control exerted by an externally applied release tape during firing.
Another process for control of registration in an LTCC structure was disclosed in U.S. Pat. No. 6,205,032 to Shepherd. The process fires a core portion of a LTCC circuit incurring normal shrinkage and shrinkage variation of an unconstrained circuit. Subsequent layers are made to match the features of the pre-fired core, which then is used to constrain the sintering of the green layers laminated to the rigid pre-fired core. The planar shrinkage is controlled to the extent of 0.8–1.2%. The technique is limited to a few layers, before registration becomes unacceptable.
As disclosed in U.S. Pat. No. 5,085,720 during the firing step to form a refractory ceramic article, a constraining tape layer or layers on the surface of the article acts to pin and restrain possible shrinkage of the article in the x and y directions due to the refractory and rigid properties of the constraining tape. The un-sintered material in the constraining tape is then removed by brushing or other cleaning procedure. The constrained material may be composed of one or more layers of green tape, which sinter and densify to form a desired ceramic LTCC body.
In the production of LTCC circuits, the use of a sacrificial constraining tape means that the user must purchase a tape material that does not end up in the final product and that it also causes a cleanliness issue (un-sintered powder removal from fired part surface) in the furnace area. Furthermore, the top and bottom surface circuitry may not be patterned and co-fired between the LTCC body and the constraining tape. In the present invention, the constraining tape is fundamentally different. It is found layered on or within the LTCC structure and becomes part of the final LTCC body.